(a) Field of the Invention
The present invention relates to a radio communication system, and in particular, to a 3-dimensional beam steering antenna system for focusing a radio beam at one spatial point so as to maximize signal transmission efficiency.
(b) Description of the Related Art
A new frequency resource, “millimeter-wave”, has a broad frequency band of 30˜300GHz and is emerging to cover future requirements of high speed multimedia communication applications.
For example, millimeter-wave systems' like LMDS and wireless LAN have been developed for 30 GHz, 60 GHz, etc. millimeter wave band communication services.
The millimeter wave technology also has important characteristics for imaging applications. The conventional imaging techniques use optical and microwave bands. If a new conceptual 3-dimensional imaging technology is developed in the millimeter wave band, it can be applied to various technical fields requiring high resolution 3-dimensional images, such as medical and engineering fields.
In spite of these merits, use of millimeter waves has a technical problem in that the output power of a millimeter wave-generating device is extremely limited because the device output power is inversely proportional to a square of a frequency according to the pf2 law, as shown in FIG. 1.
Also, compared with microwave technology, the millimeter wave technology is quite complex because of a very short wavelength, a high path loss, serious fading effects, and complex propagation characteristics which can contribute great difficulty in technology development thereof.
According to these shortcomings of the millimeter wave, received signals become weak as the frequency increases. Also, the millimeter wave technology has shortcomings in view of noise. Device gains of active devices used in a transceiver decrease and its noise factors increase, as the frequency increases. That is, the active device increases noise simultaneous with amplification of the signal. Accordingly, it is inevitable that the signal to noise ratio (SNR) is degraded when the signal passes through the active device. SNR is a significant factor for implementing radio communication systems such that the SNR should be lower than a threshold level for reliable data communication.
For overcoming these shortcomings of millimeter wave technology, a great deal of research has been undertaken around the world. However, most of it has focused on decreasing the noise at the device level and increasing the output power of the device. Another focus of research has been to decrease the interconnection loss between components and increase antenna gain by adapting a smart antenna system using a phased array.
To reduce the noise factor of the millimeter wave, however, a length of a gate should be reduced to 0.1˜0.2 μm through an e-beam lithography process, which is expensive. In spite of a reduction of the length of the gate to 0.1˜0.2 μm, SNR enhancement is only expected to an extent of about 1˜2dB. Also, to increase antenna gain, a physical size of the antenna should be increased. The increase of antenna size causes many problems in system construction, as well as increasing manufacturing costs.
Also, using superconductors can be considered for reducing loss. But in this case, the expected enhancement is still only about 1˜2dB.
The smart antenna system forms a two-dimensional beam and steers the beam, and this is a prior art technology of the 3-dimensional beam steering system of the present invention. Many research laboratories around world have devoted years of experience and research to the smart antenna system such that reception-signal-detecting antennas for a low frequency system can be commercialized so as to be used in mobile communication systems. However, the utilization of the smart antenna manufactured at the present technology level to the base station has both economical and communication quality problems. These problems may not be substantially solved using the conventional and presently-studied smart antenna technologies that depend on sensing a 2-dimensional direction. Also, a notable enhancement of the SNR of millimeter wave communication cannot be expected with this kind of approach in view of the Friis formula in which the power of the received signal is proportional to the antenna gain and inversely proportional to the square of the distance between the transmitter and receiver.